The present invention is directed to airfoil components for gas turbine engines and, more particularly, to ceramic matrix composite turbine blades, vanes and platforms.
Gas turbine engines comprise one or more rotating turbines that are used to extract energy from a high velocity and high temperature gas flow produced within the gas turbine engine. The turbines are comprised of a plurality of radially extending airfoil blades that are connected at their inner diameter ends to a rotor, which is connected to a shaft that rotates within the engine as the blades interact with the gas flow. The rotor typically comprises a disk having a plurality of axial retention slots that receive mating root portions of the blades to prevent radial dislodgment. Blades typically also include integral inner diameter platforms that prevent the high temperature gases from escaping through the radial retention slots. Between the turbine blades are disposed a plurality of radially extending stationary airfoil vanes, which are typically supported by inner and outer diameter shrouds that are suspended from an outer engine case and supported by an inner structure, respectively. During operation of the engine, the turbine blades and vanes are subjected to high heat from the gas. Additionally, the blades are subjected to high stresses from rotational forces. It is, therefore, a constant design challenge to develop materials for turbine blades and vanes that are more heat resistant to reduce cooling demands, and lighter to increase propulsive efficiencies in aircraft engines.
Typically, turbine blades and vanes are fabricated from high strength alloys as single pieces, with integral roots, platforms and shrouds. More recent turbine blade designs have attempted to incorporate ceramic matrix composite (CMC) materials, which are lightweight, heat resistant and strong. CMC material comprises a ceramic fabric that is infused with a liquid ceramic matrix. The ceramic fabric is preformed to a desired shape and the matrix solidifies within the fabric to produce a part having the lightweight and heat resistance characteristics of the matrix and the strength characteristics of the fabric. Production of thick CMC material parts is constrained because of manufacturing limitations in infusing the liquid matrix into deep layering of the preformed fabric. Inadequately infused liquid matrix produces porosity within the component that limits the heat transfer capabilities within the matrix and provides an initiation point for crack propagation. Furthermore, due to the two-dimensional nature of the ceramic fabric, difficulties arise in producing parts having complex three-dimensional shapes. For example, it is difficult to produce CMC material turbine blades having both a radially extending airfoil component and an axially extending platform component.
It is, however, desirable to use CMC material despite these complexities, as CMC materials weigh approximately one third of the weight of typical metal alloys used for turbine components, while having much higher temperature limitations. As such, production methods have been developed that attempt to overcome the aforementioned manufacturing issues. However, previous attempts at producing CMC material components have resulted in complex designs that limit the benefits of using CMC material in turbine blades. For example, one method of producing a vane involves radially stacking numerous layers of CMC material in a radial direction to obtain an airfoil shape. The stack is compressed with mechanical tensioning means to obtain the desired tensile strength and to prevent the layers from separating. Such a vane design is impractical for turbine blades because the stack extends radially in the direction in which severe stresses are generated within a rotating turbine blade. Furthermore, the mechanical tensioning means typically comprises threaded fasteners fabricated from an alloy that requires cooling, thus limiting the temperature benefits of using CMC material in a hot section of a gas turbine engine. There is, therefore, a need for improved CMC material turbine components and methods for fabricating the same.